Before I begin this review, please note that in my personal opinion, this film is a 5/5 as its various messages resonate deeply with me. However, I will instead refrain from spewing my heart out for this film and instead provide a more objective review for the benefit of those who probably just want to see a good movie.
Having said that, here's the review: 4/5 partly because Act 2 might prove to be a bit tedious to viewers; it is where our characters settle into their new roles after their well paced introductions. The good news for Act 2 is that you get to see more of Pandora and its oceans in ways you'd never even imagine. This may prove too much for some who prefer going back to the story and the action. Act 1 in contrast starts out strong and sets the stakes up perfectly while Act 3 is a grand smorgasbord of action and catharsis with a hint for an obvious sequel.
Overall, the film is a feast for the eyes, and the story is sharper and more focused. The stakes are higher for both sides and there's much more for Pandora to offer as an allegory for our own planet. The pacing may disappoint some in the middle, but it picks up dramatically, and that may not be even a problem for Avatar aficionados. Go watch it in IMAX 3D. 4DX is also worth it if you've no problem with motion sickness (it is 3 hours long after all).